Development of the vertebrate joint requires coordinated molecular and biomechanical signals during embryogenesis. Relatively few molecular pathways governing the orderly segmentation of the cartilage anlagen and the subsequent differentiation of interzone cells into synovium, tendon, and ligaments have been characterized. Clearly, members of the TGF superfamily of molecules, including GdfS, are required for joint formation. However, antagonism of other members such as Bmp2 and Bmp4 by the Bmp antagonists, Noggin and Chordin, is also critical for correct joint specification. Our studies of the LIM homeodomain (HD) transcription factor, LMXJB, in the skeletal malformation syndrome nail patella syndrome (NPS) suggest its potential involvement in a second pathway specifying joint formation. Abnormal joints and arthritis is seen in NPS, and Lrnxl b is expressed in dorsal joint mesenchyme as well as in precursors of ligaments and tendons. Moreover, Wnt7a, a member of the Wnt family of secreted morphogens has been described to regulate Lmxlb expression. In addition, we and others have found that a member of the family of Wnt protein antagonists, secreted Frizzled-related protein 2 (sFrp2), is also expressed specifically in joint mesenchyrne. Analogous to the critical regulation of Tgf proteins, we hypothesize that another regulatory circuit involving Wnt and Wnt antagonists may be critical for specifying joint morphogenesis. In addition, Lmxl b may be one of the intracellular components which may bridge these components. We propose to elucidate the importance of these molecules during joint development by determining their roles and their candidate targets. Specifically, the potential downstream targets for Lmxlb action will be determined by comparing expression profiles of Lmxl b-I- and Lrnxlb+/+ mouse limbs. Second, the in vivo role of sFrp2 function will be correlated with its in vitro pattern of expression by generation of targeted null alleles in mice. Finally, the phenotypic consequences of sFrp2 null mutations on the molecular markers of joint development will be correlated with potential mutations in several inherited synostotic syndromes. These studies should lend insight to the molecular pathways which govern embryonic joint development as well as the consequences of their dysregulation in birth defects.